THIS INVENTION relates to a method of producing secondary metabolites, and also to the treatment of waste water in a bioreactor by utilising the capability of certain micro-organisms to produce secondary metabolites.
In the presence of a nutrient solution of sufficiently high concentration, most micro-organisms exhibit exponential growth. This is also referred herein to as primary growth. As the concentration of the nutrient solution falls, the micro-organisms, in response to the stress caused by nutrient starvation, switch to what is referred herein to as secondary metabolism in which they start to produce secondary metabolites.
Certain secondary metabolites have useful properties. Phanerochaete chrysosporium, for example, is a filamentous fungus capable of degrading a wide range of recalcitrant aromatic pollutants. These compounds include BTEX (Benzene, Toluene, Ethylbenzene and Xylene) type compounds, DDT, TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin), benzo(a)pyrene, Lindane and certain PCB congeners. This organism has thus been considered a candidate for the bioremediation of waste waters containing such pollutants.
This degradative ability is due in part to the secretion, during secondary metabolism initiated by nutrient limiting conditions, of a group of H.sub.2 O.sub.2 -producing oxidases as well as a group of peroxidases called lignin peroxidases (LiP). In whole cell cultures, however, a certain amount of biodegradation of these compounds occurs independently of the secretion of these enzymes.
Several problems have been encountered in attempting to achieve efficient production of the ligninolytic enzymes from this fungus, and in employing the fungus for waste water treatment making use of conventional bioreactor technology. Existing methods of producing secondary metabolites for extended periods of time are normally sequential batch processes. The processes are often slow and the yields low, and difficulties have been experienced in attempting to upscale the processes for large-scale commercial use. In each cycle of the batch process there is a long lag phase. Thereafter there is a primary growth phase lasting approximately 3 days, after which the secondary metabolism commences. The enzymes of interest are normally produced after 5 days, and production normally ceases after 7 days. After approximately 8 days the micro-organisms start to produce spores and die.
It is an object of the present invention to provide a method of producing secondary metabolites which can operate on a continuous basis.